Much of the crude still in the Gulf and coastal areas more than three months after BP's blowout has permeated deep into marshes and wetlands, complicating cleanup.
Crews are still finding plenty of crude in those interior areas, even as government officials say spotting oil from the air on the Gulf's surface is taking longer on each trip.
"The good news is people are seeing less oil, but the bad news is the oil trapped in the marshes is moving out with the tides and sticking on the marsh cane," said Maura Wood, an oceanographer with the National Wildlife Foundation, on a boat trip to the marshes of Pass-A-Loutre, La. "And that could kill it."
The sometimes frustrating search for oil underscores the difficulties facing the small army of federal officials and cleanup crews tasked with purging what remains. Rear Adm. Paul Zukunft, the government's on-scene coordinator, said he's had to spend a growing amount of his time taking flights over the Gulf to search for the remaining crude.
"There is very little observable oil out there," he said, saying that Coast Guard responders are not seeing much on the surface. But he added: "We can't turn a blind eye ... If we don't see oil, I'm not assuming it doesn't exist."
Engineers, meanwhile, were working to make sure no new oil would seep from the busted well. They scored another victory Thursday by finishing the pumping of a steady stream of fresh cement down the throat of the well, and crews planned to wait at least a day for it to dry.
The cement was one of the last steps in the so-called "static kill." The effort started Tuesday with engineers pumping enough mud down the top of the well to push the crude back to its underground source for the first time since an oil rig exploded 50 miles off the Louisiana coast on April 20, killing 11 workers and triggering the spill.
Crews followed it up Thursday by sealing the well with a torrent of cement. After it dries, the last step begins: Finishing the drilling of the last 100 feet of the relief well, which government officials said will be used to seal the underground reservoir from the bottom with mud and cement.
"This is not the end, but it will virtually assure us that there will be no chance of oil leaking into the environment," retired Adm. Thad Allen, who oversees the spill response for the government, said in Washington.
The progress was another bright spot as the tide appeared to be turning in the monthslong battle to contain the oil, with a federal report this week indicating that only about a quarter of the spilled crude remains in the Gulf and is degrading quickly.
Despite the progress on the static kill, BP executives and federal officials won't declare the threat dashed until they use the relief well - though lately they haven't been able to publicly agree on its role.
Federal officials including Allen have insisted that crews will shove mud and cement through the 18,000-foot relief well, which should be completed within weeks. Crews can't be sure the area between the inner piping and outer casing has been plugged until the relief well is complete, he said.
But for reasons unclear, BP officials have in recent days refused to commit to pumping cement down the relief well, saying only that it will be used in some fashion. BP officials have not elaborated on other options, but those could include using the well simply to test whether the reservoir is plugged.
The vast oil reservoir beneath the well could still be worth billions of dollars even after it spewed crude into the Gulf of Mexico for more than three months, but BP isn't saying whether it plans to cash in on this potential windfall.
BP insisted Thursday it had no plans to use it or its two relief wells to produce oil. But the company won't comment on the possibility of drilling in the same block of sea floor someday or selling the rights to the entire tract to another oil company.
Whether the well is considered sealed yet or not, there's still oil in the Gulf or on its shores - nearly 53 million gallons of it, according to the report released Wednesday by the Interior Department and the National Oceanic and Atmospheric Administration.
That's still nearly five times the size of the Exxon Valdez spill, which wreaked environmental havoc in Alaska in 1989.
But almost three-quarters of the nearly 207 million gallons of oil that leaked overall has been collected at the well by a temporary containment cap, been cleaned up or chemically dispersed, or naturally deteriorated, evaporated or dissolved, the report said.
Some residents are worried that now that the well has flatlined, the nation's attention will shift from the coast.
"I'm losing trust in the whole system," said Willie Davis, a 41-year-old harbormaster in Pass Christian, Miss. "If they don't get up off their behinds and do something now, it's gonna be years before we're back whole again."
In Pass-A-Loutre, where oil still clung stubbornly to marsh cane, each day's high tide picks up the goo and leaks it back into the ocean. But Jeremy Ingram, the Coast Guard official who oversees cleanup crews here, said it's cleaner than it was when he arrived 60 days ago. Back then, he said, he couldn't even see water through the thick ooze.
"I'd say it's a lot less than what was here, but if you see on the canes it's still heavily saturated with oil. So the job's not done yet, there's still a lot more work to get done," he said. "As the tide comes up and washes oil off that cane, somebody and some thing has to be here to catch it."
This program aired on August 6, 2010. The audio for this program is not available.